Observational Analysis of a Wind Gust Event during the Merging of a Bow Echo and Mini-Supercell in Southeastern China

The merging of a fast-moving bow echo with a convective cell of a hook-echo signature was studied by using polarimetric radar detections. Gusts with wind speeds near 35 m s–1 were recorded by the surface station, which caused significant damage. A convective cell with a mesovortex signature, which is hereafter referred to as a mini-supercell, was observed over the northeast of the bow echo before the convective merging. It was found that the mesovortex possessed cyclonic circulation and resembled a supercell-like feature. The merging of the bow echo and the mini-supercell strengthened the updraft near the apex of the bow echo. The enhanced updraft was also demonstrated by the appearance of a differential reflectivity (ZDR) column with a topmost height of 4 km above the melting layer (~4 km). The bow was separated into northern and southern sectors after merging with the mini-supercell, leading to the gusty wind over the surface of the south sector.

Differences between Severe and Nonsevere Warm-Season, Nocturnal Bow Echo Environments

Nocturnal bow echoes can produce wind damage, even in situations where elevated convection occurs. Accurate forecasts of wind potential tend to be more challenging for operational forecasters than for daytime bows because of incomplete understanding of how elevated convection interacts with the stable boundary layer. The present study compares the differences in warm-season, nocturnal bow echo environments in which high intensity [>70 kt (1 kt ≈ 0.51 m s−1)] severe winds (HS), low intensity (50–55 kt) severe winds (LS), and nonsevere winds (NS) occurred. Using a sample of 132 events from 2010 to 2018, 43 forecast parameters from the SPC mesoanalysis system were examined over a 120 km × 120 km region centered on the strongest storm report or most pronounced bowing convective segment. Severe composite parameters are found to be among the best discriminators between all severity types, especially derecho composite parameter (DCP) and significant tornado parameter (STP). Shear parameters are significant discriminators only between severe and nonsevere cases, while convective available potential energy (CAPE) parameters are significant discriminators only between HS and LS/NS bow echoes. Convective inhibition (CIN) is among the worst discriminators for all severity types. The parameters providing the most predictive skill for HS bow echoes are STP and most unstable CAPE, and for LS bow echoes these are the V wind component at best CAPE (VMXP) level, STP, and the supercell composite parameter. Combinations of two parameters are shown to improve forecasting skill further, with the combination of surface-based CAPE and 0–6-km U shear component, and DCP and VMXP, providing the most skillful HS and LS forecasts, respectively.

Severe Convective Windstorms in Europe: Climatology, Preconvective Environments, and Convective Mode

The frequency of European convective windstorms, environments in which they form, and their convective organizational modes remain largely unknown. A climatology is produced using 10 233 severe convective-wind reports from the European Severe Weather Database between 2009–2018. Severe convective-wind days have increased from 50 days yr–1 in 2009 to 117 days yr–1 in 2018, largely because of an increase in reporting. The highest frequency of reports occurred across central Europe, particularly Poland. Reporting was most frequent in summer, when a severe convective windstorm occurred every other day on average. The preconvective environment was assessed using 361 proximity soundings from 45 stations between 2006–2018, and a clustering technique was used to distinguish different environments from nine variables. Two environments for severe convective storms occurred: Type 1, generally low-shear–high-CAPE (mostly in the warm season) and Type 2, generally high-shear–low-CAPE (convective available potential energy; mostly in the cold season). Because convective mode often relates to the type of weather hazard, convective organizational mode was studied from 185 windstorms that occurred between 2013–2018. In Type-1 environments, the most frequent convective mode was cells, accounting for 58.5% of events, followed by linear modes (29%) and the nonlinear noncellular mode (12.5%). In Type-2 environments, the most frequent convective mode was linear modes (55%), followed by cells (36%) and the nonlinear noncellular mode (9%). Only 10% of windstorms were associated with bow echoes, a much lower percentage than other studies, suggesting that forecasters should not necessarily wait to see a bow echo before issuing a warning for strong winds.

A 10-Year Radar-Based Climatology of Mesoscale Convective System Archetypes and Derechos in Poland

In this study, a 10-yr (2008–17) radar-based mesoscale convective system (MCS) and derecho climatology for Poland is presented. This is one of the first attempts of a European country to investigate morphological and precipitation archetypes of MCSs as prior studies were mostly based on satellite data. Despite its ubiquity and significance for society, economy, agriculture, and water availability, little is known about the climatological aspects of MCSs over central Europe. Our results indicate that MCSs are not rare in Poland as an annual mean of 77 MCSs and 49 days with MCS can be depicted for Poland. Their lifetime ranges typically from 3 to 6 h, with initiation time around the afternoon hours (1200–1400 UTC) and dissipation stage in the evening (1900–2000 UTC). The most frequent morphological type of MCSs is a broken line (58% of cases), then areal/cluster (25%), and then quasi-linear convective systems (QLCS; 17%), which are usually associated with a bow echo (72% of QLCS). QLCS are the feature with the longest life cycle. Among precipitation archetypes of linear MCSs, trailing stratiform (73%) and parallel stratiform (25%) are the most common. MCSs are usually observed from April to September, with a peak in mid-July. A majority of MCSs travels from the west, southwest, and south sectors. A total of 16 derecho events were identified (1.5% of all MCS and 9.1% of all QLCS); the majority of them were produced by a warm-season QLCS, whereas only 4 were produced by cold-season narrow cold-frontal rainbands. Warm-season derechos produced a bigger impact than did cold-season events, even though their damage paths were shorter.

Composite In Situ Microphysical Analysis of All Spiral Vertical Profiles Executed within BAMEX and PECAN Mesoscale Convective Systems

Vertical profiles of temperature, relative humidity, cloud particle concentration, median mass dimension, and mass content were derived using instruments on the NOAA P-3 aircraft for 37 spiral ascents/descents flown within five mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) project, and 16 spiral descents of the NOAA P-3 within 10 MCSs during the 2003 Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The statistical distribution of thermodynamic and microphysical properties within these spirals is presented in context of three primary MCS regions—the transition zone (TZ), enhanced stratiform rain region (ESR), and the anvil region (AR)—allowing deductions concerning the relative importance and nature of microphysical processes in each region. Aggregation was ubiquitous across all MCS zones at subfreezing temperatures, where the degree of ambient subsaturation, if present, moderated the effectiveness of this process via sublimation. The predominately ice-supersaturated ESR experienced the least impact of sublimation on microphysical characteristics relative to the TZ and AR. Aggregation was most limited by sublimation in the ice-subsaturated AR, where total particle number and mass concentrations decreased most rapidly with increasing temperature. Sublimation cooling at the surface of ice particles in the TZ, the driest of the three regions, allowed ice to survive to temperatures as high as +6.8°C. Two spirals executed behind a frontal squall line exhibited a high incidence of pristine ice crystals, and notably different characteristics from most other spirals. Gradual meso- to synoptic-scale ascent in this region likely contributed to the observed differences.

Damaging Convective and Non-Convective Winds in Southwestern Iberia during Windstorm Xola

On 23/12/2009, windstorm Xola struck mainland Portugal, causing serious damage in a small area north of Lisbon (Oeste region) and in the south region, inflicting economic losses of over EUR 100 million. In both areas, several power towers, designed to withstand up to 46 m s−1 winds, were destroyed. The causes of these two distinct damaging wind events were investigated. Xola was revealed to have a prominent cloud head and a split cold front structure. In the southern region, the damages were due to downburst winds, associated with a mesovortex, observed in a bow echo line triggered by an upper cold front. The cloud head presented several dry air intrusion signatures, co-located with tops progressively lowering towards the hooked tip. This tip revealed features consistent with the presence of slantwise convection, the descending branches of which may have been strengthened by evaporating cooling. At the reflectivity cloud head tip, a jet streak pattern was identified on weather radar, with Doppler velocities exceeding 55 m s−1, just 400 m above ground. This signature is coherent with the presence of a Sting jet, and this phenomenon was associated with the strongest wind gusts (over 40 m s−1) and the largest damages in the Oeste region.

Sensitivity of a Bowing Mesoscale Convective System to Horizontal Grid Spacing in a Convection-Allowing Ensemble

The bow echo, a mesoscale convective system (MCS) responsible for much hail and wind damage across the United States, is associated with poor skill in convection-allowing numerical model forecasts. Given the decrease in convection-allowing grid spacings within many operational forecasting systems, we investigate the effect of finer resolution on the character of bowing-MCS development in a real-data numerical simulation. Two ensembles were generated: one with a single domain of 3-km horizontal grid spacing, and another nesting a 1-km domain with two-way feedback. Ensemble members were generated from their control member with a stochastic kinetic-energy backscatter scheme, with identical initial and lateral-boundary conditions. Results suggest that resolution reduces hindcast skill of this MCS, as measured with an adaptation of the object-based Structure–Amplitude–Location method. The nested 1-km ensemble produces a faster system than in both the 3-km ensemble and observations. The nested 1-km simulation also produced stronger cold pools, which could be enhanced by the increased (fractal) cloud surface area with higher resolution, allowing more entrainment of dry air and hence increased evaporative cooling.